[SYCL][DOC] Update C-CXX-StandardLibrary doc to align with latest status (#2529)

Signed-off-by: gejin <[email protected]>

Author: jinge90

sycl/doc/extensions/C-CXX-StandardLibrary/C-CXX-StandardLibrary.rst

@@ -10,27 +10,7 @@ explicitly included in user code.
 
 Implementation requires a special device library to be linked with a
 SYCL program. The library should match the C or C++ standard library
-used to compile the program:
-
-For example, on Linux with GNU glibc:
-.. code:
-   clang++ -fsycl -c main.cpp -o main.o
-   clang++ -fsycl main.o $(SYCL_INSTALL)/lib/libsycl-glibc.o -o a.out
-
-or, in case of Windows:
-.. code:
-   clang++ -fsycl -c main.cpp -o main.obj
-   clang++ -fsycl main.obj %SYCL_INSTALL%/lib/libsycl-msvc.o -o a.exe
-
-For Ahead-Of-Time compilation (AOT), fallback libraries (object files)
-must be linked as well:
-
-.. code:
-   clang++ -fsycl -c  -fsycl-targets=spir64_x86_64-unknown-unknown-sycldevice \
-       main.cpp -o main.o
-   clang++ -fsycl -fsycl-targets=spir64_x86_64-unknown-unknown-sycldevice \
-       main.o $(SYCL_INSTALL)/lib/libsycl-glibc.o                         \
-              $(SYCL_INSTALL)/lib/libsycl-fallback-cassert.o -o a.out
+used to compile the program.
 
 List of supported functions from C standard library:
   - assert macro          (from <assert.h> or <cassert>)
@@ -120,12 +100,38 @@ following math functions are not supported now:
  - lrintf, lrint
  - nexttowardf, nexttoward
  - nanf, nan
+
 Device libraries can't support both single and double precision as some
 underlying device may not support double precision.
 'ldexpf' and 'frexpf' from MSVC <math.h> are implemented using corresponding
 double precision version, they can be used only when double precision is
 supported by underlying device.
 
+All device libraries without double precision usage are linked by default and
+in order to use double precision device library, please add
+'-fsycl-device-lib=libm-fp64' or '-fsycl-device-lib=all'.
+
+For example, no options need to be added to use `assert` or float precision
+math functions:
+.. code:
+   clang++ -fsycl main.cpp -o main.o
+
+To use double precision math functions:
+.. code:
+   clang++ -fsycl -fsycl-device-lib=libm-fp64 main.cpp -o main.o
+
+For Ahead-Of-Time compilation (AOT), the steps to use device libraries is
+same, no options need to be added to use `assert` or float precision math
+functions:
+.. code:
+   clang++ -fsycl -fsycl-targets=spir64_x86_64-unknown-unknown-sycldevice \
+       main.cpp -o main.o
+
+To use double precision math functions in AOT:
+.. code:
+   clang++ -fsycl -fsycl-device-lib=libm-fp64 -fsycl-targets=spir64_x86_64-unknown-unknown-sycldevice \
+       main.cpp -o main.o
+
 Example of usage
 ================
 
@@ -212,11 +218,7 @@ wrapper libraries are provided with the SYCL compiler that "lower"
 libc implementation-specific functions into a stable set of functions,
 that can be later handled by a device compiler.
 
-.. code:
-   clang++ -fsycl -c main.cpp -o main.o
-   clang++ -fsycl main.o $(SYCL_INSTALL)/lib/libsycl-glibc.o -o a.out
-
-This `libsycl-glibc.o` is one of these wrapper libraries: it provides
+This `libsycl-crt.o` is one of these wrapper libraries: it provides
 definitions for glibc specific library function, and these definitions
 call the corresponding functions from `__devicelib_*` set of
 functions.
@@ -237,11 +239,7 @@ For example, `__assert_fail` from IR above gets transformed into:
       unreachable
 
 A single wrapper object provides function wrappers for *all* supported
-library functions. Every supported C library implementation (MSVC or
-glibc) has its own wrapper library object:
-
-  - libsycl-glibc.o
-  - libsycl-msvc.o
+library functions.
 
 SPIR-V
 ======
@@ -283,4 +281,5 @@ extension is provided as `libsycl-fallback-cassert.spv`
 For AOT compilation, fallback libraries are provided as object files
 (e.g. `libsycl-fallback-cassert.o`) which contain device code in LLVM
 IR format. Device code in these object files is equivalent to device
-code in the `*.spv` files.
+code in the `*.spv` files. Those object files are located in compiler
+package's 'lib/' folder.

sycl/source/detail/program_manager/program_manager.cpp

@@ -388,9 +388,6 @@ RT::PiProgram ProgramManager::getBuiltPIProgram(OSModuleHandle M,
     // If device image is not SPIR-V, DeviceLibReqMask will be 0 which means
     // no fallback device library will be linked.
     uint32_t DeviceLibReqMask = 0;
-    // FIXME: disable the fallback device libraries online link as not all
-    // backend supports spv online link. Need to enable it when all backends
-    // support spv online link.
     if (Img.getFormat() == PI_DEVICE_BINARY_TYPE_SPIRV &&
         !SYCLConfig<SYCL_DEVICELIB_NO_FALLBACK>::get())
       DeviceLibReqMask = getDeviceLibReqMask(Img);
@@ -797,11 +794,11 @@ ProgramManager::ProgramPtr ProgramManager::build(
     LinkOpts = LinkOptions.c_str();
   }
 
-  // TODO: Because online linking isn't implemented yet on Level Zero, the
-  // compiler always links against the fallback device libraries.  Once
-  // online linking is supported on all backends, we should remove the line
-  // below and also change the compiler, so it no longer links the fallback
-  // code unconditionally.
+  // TODO: Currently, online linking isn't implemented yet on Level Zero.
+  // To enable device libraries and unify the behaviors on all backends,
+  // online linking is disabled temporarily, all fallback device libraries
+  // will be linked offline. When Level Zero supports online linking, we need
+  // to remove the line of code below and switch back to online linking.
   LinkDeviceLibs = false;
 
   // TODO: this is a temporary workaround for GPU tests for ESIMD compiler.